Among the strains of STEC, seven serogroups, i.e., O157, O26, O45, O103, O111, O121, and O145, have been identified to be associated with severe illness in humans. These serogroups are referred to in the following as STEC Top7. The United States Department of Agriculture (USDA) has implemented regulations applicable to the analysis of certain raw beef products which define in addition to the serogroup O157 six further serogroups as identified above as adulterants.
When determining the presence or absence of these strains of STEC, typically a food sample is added to a culture medium enabling growth of E. coli to provide an E. coli stock medium.
Subsequently, lysis of the E. coli in the stock medium provides for a sample solution comprising E. coli DNA.
The subsequent detection of the above-mentioned serogroups in the samples in a reliable manner using conventional methods is a time consuming procedure.
The use of polymerase chain reaction (PCR) for detecting the major STEC virulence genes stx1, stx2, eae, and serogroup-specific genes is set out in the ISO 13136 technical specification (ISO/TS 13136:2012). PCR positive samples have to be confirmed by cultural methods as the different genes could belong to different strains, leading to false positive results.